Control system



i y D. R. SHOULTS ET AL, 2,250,209

CONIROLSYSTEM Filed Nov. 17, 1939 2 Sheets-Sheet 1 Fig.1.

Inventor:

David R Shoults,

b William D. Coc reH,

His Attorne July 22, 1941. D. R. SHOULTS ET AL 2,250,209

CONTROL SYSTEM Filed Nov. 17, 1939 2 Sheets-Sheet 2 IT'IVGT'WLOT: DavidR. Shoults, William D. Co'ckrell,

His Attorneg- Patented July 22, 1941 CONTROL SYSTEM David a. Shoults andWilliam D. Coolu'ell, Schenectady, N. Y., asslgnors to General ElectricCompany, a corporation of New York Application November 17. 1939, SerialNo. 304,996

17 Claims.

This invention relates to control systems, more particularly to systemsfor controlling the relative positions of a length of moving materialand an element operating thereon so that the element performs itsoperations in register with marks, designs, or other matter which recurat predetermined intervals on the length of material, and an object ofthe invention is the provision of a reliable and improved system of thischaracter.

Heretofore register control systems have been used in which registermarks or indicia have been printed on a strip of material at intervalsof from several inches to several feet, and in which these marks werescanned by a light sensitive scanning head as the strip of material was.fed to the operating element. In such systems, when a register markpasses the scanning head, at such a time as to indicate an error inregister between the strip and the operating element, a correction ismade in the register of a predetermined provision of a. register controlsystem having much greater accuracy than the register control systemsused heretofore.

In carrying the invention into effect in one form thereof, lightsensitive means are provided for responding .to reference marks orindicia on a length of moving material to produce an electrical impulsein accordance with the position of thematerial, and means are providedfor producing an electrical impulse in accordance with the position ofthe element which is to operate on the material. Means responsive toboth the im pulses produced by the material and by the operating elementare provided which correct any error in the register of the operatingelement with respect to the length of moving material by consmallamount, such for example, as of aninch. During the time this correctionis being made the equipment is not responsive to any further controlimpulses. As it requires approximately one-half second in most cases,with contactor control to accomplish any substantial correcting movementof the strip of material, this means that the correcting speed isapproximately of an inch per second or approximately 4 inches perminute. If the accuracy to be held is closer than g of an inch, then thecorrection that is made in response to a register mark must be madesmaller and this requires that the cork 'recting speed must becorrespondinglyreduced.

There is then, a more or less fundamental limitation in the correctingspeed which can be obtained with the type of register control apparatusused heretofore.

In the control system of the present invention, the register marks orindicia are spaced closely together on the strip so that the number ofimpulses per second is much greater than the number per second in theequipment used heretofore. For example, the spacing of the registermarks in the'system of the present invention may be such that the numberof impulses will be greater than 40 per second, and the correction thatis being applied at any instant will depend upon a summation of pastindications of error and also upon the corrections, which have beenapplied. Thus instead of operating on an intermittent basis as thesystems heretofore used have operated, the control system of the presentinvention will in efiect be a continuously functioning device, and afurther object of the invention is the tinuously varying their relativepositions, as long as any error exists.

In order to produce this continuous correction, a suitable electricalenergy storage devicev is provided. When an error exists in the registerof the operating element with respect to the length of material, theelectrical impulses produced by the length of material and by theelement produce a variation in the amount of electrical energy stored inthe energy storage device, and means responsive to the amount of energystored in the device are provided for correctin the error in register.

In illustrating the invention in one form. thereof, it is shown asembodied in a system for controlling the register between ainovinglength of material on which the faces of postage stamps have beenprinted and a perforating element and an element for severing sheets ofstamps from the advancing length after perforation thereof. It will beunderstood, however, that the invention has other applications.

For a better and more complete understanding of the invention, referenceshould now be had to the following specification and to the accompanyingdrawings of which Fig. l is a simple, diagrammatical sketch of anembodiment of the invention and Fig. 2 is a simple, diagrammatical viewin perspective of certain elements of the invention and certain elementsof a stamp perforating machine, and serves to illustrate the applicationof the invention to the perforating ma chine.

Referring now to the drawings, a length of material in having 'the faces(not shown) of postage stamps previously printed thereon is unwound froma roll (not shown) and fed by means of feed rolls H in the direction ofarrow i2 to the perforating element I3 and to the sheet severing elementi4. The feeding element H comprising top and bottom roll which grip thematerial tightly is driven by suitable driving means, such for example,as the electric motor I5 which is supplied from a suitable source (notshown). It will be understood that any suitable means may be providedfor varying the speed of motor l5 so that the perforating machine may beoperated at a. desirable speed.

The perforating cylinders 13 are connected to motor 15 through amechanical differential device is, bevel gearing i1, and shaft IS. Thering gear 169. of the differential device is driven by a small directcurrent motor i9 to which it is connected by means of worm gearing 20.The ratio of the gearing between the drive motor l5 and perforatorcylinders I3 is such that when the ring gear IE8 is stationary, theperforator rolls run 1% (or other selected value) faster than thefeeding element ll and when the ring gear is being driven at full speedin the proper direction, the perforator rolls run 1% (or other selectedvalue) slower than the feeding element.

The motor i9 receives armature power from a small direct currentgenerator 2i driven from the machine drive shaft so that the output ofthis generator and the consequent speed of the correcting motor is isproportional to the value of the field excitation of the generator andalso to the speed of the machine.

As shown in Fig. 1, the field winding Zia of generator 222 is suppliedfrom the direct current bus 22 through a plurality of high vacuumampliier valves 23, 2i, and 25. The circuit is traced as follows: fromthe top line of bus :22, through conductor 23, field winding 2 is,switch it? (in the right-hand position thereof), through theanodecathcde circuits of valves 23, 24, and 35 in parallel, and thenceby conductors 28, 29, and to the mid or grounded side of bus 22.

The control grids of these amplifier valves are in turn energized from avoltage amplifying tube 3; which is controlled by register conditions ofthe perforating element i3 with respect to web ID.

The control system is provided with a scanning head which includes asource of light 329., a light sensitive device 32b such as aphotoelectric cell, and an amplifier tube 33. This scanning head detectsthe passage of indicia or register marks 34 printed on the web. A secondscanning head 35 having two light sensitive devices such asphotoelectric cells 36 and 37, and amplifier tubes 38 and 39,respectively, observes the motion of a disc 40 which is attached to theperforating cylinder l3. This disc has a series of slots cut in it, inexact relationship with the register marks on the length of material I0when the material is being fed into the machine under proper conditionsfor correct perforation. A lamp 4| projects a beam of light through eachslot on disc 40 as it passes between the lamp and scanning head 35.Scanning head 35 is provided with two slits 36a and 31a and also withtwo prisms 36b and 31b. Prisms 36b and 31s are so arranged that lightbeams from source 4| entering slits 36a and 31s are directed onphotoelectric cells 33 and 31, respectively. The slits 36,. and 31s areso positioned that under conditions of correct register light falls onone of the photo tubes in the double scanning head just before and onthe other photo tube just after the instant that a register mark 34passes beneath scanning head 32 and darkens the photo cell 32%.Preferably the slits 36; and 31s are so positioned that the image of oneof the slots of disc 40 as magnified by lens 42 is slightly narrowerthan the space between slits 38. and 31.. Thus there is a very briefinterval or "dead zone" at the correct time of arrival of a registermark beneath scanning head 32, this interval being equal to the timerequired for the register mark 34 to pass beneath the scanning head 32.

Preferably the register marks 34 on material I0 occur very closetogether, e. g., at intervals of approximately one inch, and the numberof slots on disc 43 is such that one slot passes between source 4| andscanning head 35 each time a register mark 34 passes beneath scanninghead 32. The control is so arranged that if the image of a slot in thedisc 40 falls between the slits 36a and 31- during the instant that aregister mark 34 passes beneath scanning head 32 and darkens photo cell32b, no correcting impulse will be passed to the correcting mechanism.

On the other hand, if during the time the register mark 34 darkens thephoto tube 32s, the light image of a slot of disc 40 overlaps one of theslits 359. or 315 of scanning head 35 by a minute amount, e. g., anamount corresponding to .001 inch movement of the Web, an electricalimpulse will be passed through one or the other of mixing valves 44 and46 to an energy storage oevice 46, illustrated as comprising a condenser41 of relatively high capacitance and a condenser 48 of relatively lowcapacitance connected in series relationship. Condenser 41 is shunted bya high resistance resistor 49 and therefore its circuit has a relativelylong time constant, and condenser 48 is shunted by a relatively lowresistance resistor 49:. and therefore its circuit has a short timeconstant.

Alternating current power is supplied from the alternating current lineto the primaries of transformers 5G and Si which supply lamps, heatersand rectifier supply. Direct current for operation of the amplifiervalves is obtained through rectifiers, and this direct current isfiltered by reactor 53 and capacitors 34. The transformer and rectifiermay be designed to produce any suitable value of direct current voltage.For example, this voltage may be 350 volts and may be applied to asuitable voltage divider comprising resistor sections 55, 56, and 51.The point between resistor sections 55 and 56 may be grounded asindicated at 53 so that conductor 38 is a. desired voltage above groundpotential, e. g. 200 volts, and conductor 30 is a desired value belowground potential, e. g. volts.

The anode of amplifier tube 33 is connected through resistor 6| toconductor 59 and the connection point 3311. between the anode andresistor is connected through a condenser 52 to the grids 4a and 45. ofmixer valves 44 and 43.

Similarly, the anode of amplifier valve 33 is connected through a.resistor 63 to conductor 33, and the connection point 33. is connectedthrough condenser 64 to the grid 44s of mixing valve 44. Likewise theanode of amplifier valve 39 is connected to the supply conductor 39through resistor 35 and its connection point is connected throughcondenser 65 to the grid 45b of mixing valve 45.

The voltages applied to the grids of amplifier valve 33 are such thatthe valve normally conducts current when no register mark is beneathscanning head 32. When a register mark passes beneath scanning head 32and darkens photo cell 32s, the voltage applied to grid 33b decreasesmomentarily and the current flowing in the anode-cathode circuit isreduced abruptly. As a result the IR drop across resistor 6| decreasesand the voltage of point 33. rises sharply, and as soon as the registermark has passed, the voltage oi point 33. falls abruptly to its formervalue. Consequently, positive voltage impulse is transmitted throughcondenser 82 to grids 64a and 45-. of mixing valves 44 and 45.

, Likewise, the voltages applied to the grids of valves 38 and 33 aresuch that the valves are conducting when no light from source 4! enterseither slit 38. or 3h. In this connection it will be noted that photocells 36 and 31 are connected oppositely in circuit withgrids 33p and39p of amplifier valves 33 and 33 as compared with the connection ofphotocell 32b and the grid 33 of amplifier valve 33. Consequently whenlight from source 4| enters slit 38s or slit 31a and falls upon photocell 36 or upon photo cell 31', the current flowing in the anode-cathodecircuit of amplifier valve 38 or amplifier valve 39 decreases abruptlyand the voltage of the point 38s. or point 39:. rises abruptly. Whenlight from the source 4i ceases to fall upon the cells 36 and 31, thevoltages oi points 388 and 33 decrease rapidly to their normal values.Consequently an electrical impulse is transmitted through condenser 34or condenser 63 to the grids 44s or 45b of mixing valves 44 or 45.

These two valves are pentodes arranged for suppressor grid control sothat if either the suppressor grid or the control grid is sufilcientlynegative no current will flow in the anode circuit. Therefore, it isnecessary that light from source 4| enter one of the slits 3'52. or 31sand that simultaneously one of the register marks 34 should pass underscanning head 32 so as to darken photo cell 32b in order that either ofthese mixing valves may be rendered conducting.

In order that the mixing valve 44 may cause a positive impulse ofcurrent to be supplied to energy storage device 48, the connection point.e oi its anode with resistor 61 is connected to the control grid of avalve 68 which is connected as an inverter valve to invert the negativeimpulse from valve 44 and apply a positive impulse to energy storagedevice 48. The voltages applied to the grids of valve 38 are such thatthe valve is normally conducting, i. e., current is flowing in its anodecircuit.

With the foregoing understanding of the elements and their organizationin the complete control system, the operation of the system itseli willreadily be understood from the following description.

It is assumed that the motor I is energized and is driving the feedrolls H, the perforating cylinders l3, and cutofl cylinders I4- Theperforating cylinders andthe cutoif cylinders are driven at a speedwhich would be 1% faster than the speed ofthe material I0 if thecorrecting member I6; of the differential device were moti0nless..Switch 21 is closed to the right to connect the field 2's. of generator2| to the anode circuits of valves 23, 24 and 25. a

The movable contact 69 is connected through resistors 49 and 49a to thegrid oi? the amplifier valve 3| which controls the valves 23, 24, and 23which supply the excitation current to the generator 2| from whichcorrecting motor I. is supplied. Consequently the position of movablecontact 63 on the section 51 of the voltage divider controls the speedof the correcting motor i9. Movable contact 69 is moved to a. po-

sition on section 51 of the voltage divider so that the correcting motordrives the correcting element i3; at a speed such that the speed of theperforating cylinders I3 is equal to the speed 01 material III withnormal conditions of web tension and printing. In other words, the motorl3 operates at approximately half speed.

I! the perforating cylinders l3 are perforating in correct register withthe stamp face designs on material ill, the light image of each slot ondisc 40 lies between the two slits 36a and 31; at the instant that oneof the register marks "34 is in position beneath scanning head 32 todarken the photo cell 32b. Consequently, the electrical impulsetransmitted by photo cell 32o through amplifier 33 and condenser 52 tothe grids 44s and 5a of mixing valves 44 and 45 in response to a spot 34is not applied to the grids simultaneously with the electrical impulsestransmitted by photo cells 38 and 31, through amplifier valves 38 and 39and through condensers B4 and 33 to the grids 44b and 5b. Since theimpulses received on the grids of each mixing valve are not receivedsimultaneously, these mixing valves remain non-conducting and noelectrical impulse is transmitted to the electrical energy storagedevice 43. As long as this condition of correct register obtains, thegrid voltage of amplifier valve 3! remains unchanged and the speed ofthe correcting motor i3 which is ultimately controlled by amplifiervalve 3! re mains unchanged.

However, ii the perforating cylinders it should become out of registerwith the stamp faces on material ill by a minute amount, e. g., .001in., the impulses applied to the grids of one or the other of the mixingvalves 44 or 45 will be simultaneous; one or more electrical impulses ofduration proportional to the amount of misreg ister will be transmittedto the electrical energy storage device 43, and the speed of thecorrecting motor will be changed.

For example, assume that for some reason such as shrinkage of thematerial ID, the periorator rolls i3 become slow with respect to thedesign on material [0 so that an out of register condition arises. Assoon as an extremely small amount of this out oi register condition, e.g., .001 in., is present, some light from source 4i will pass through aslot on disc 40 and enter slit 36a and fall upon photo cell 36 at theexact instant that a register mark 34 passes beneath scanning head 32and darkens photo cell 32b. Consequently the electrical impulsestransmitted by photo cells 32a and 36 will be received simultaneously onthe grids 4a. and 44b of mixing valve 44. As a result of thesimultaneous application of control voltages to its grids, valve 44becomes conducting. The current increases abruptly in the anode circuitof valve 44 and also ceases abruptly after a slight movement of theregister mark 34 or the slot in the disc 40 and thus results in animpulse of current of extremely brief duration. This current impulselowers the voltage of connection point 44c which lowers the potential ofgrid 68a of inverter valve 88. As a result the current normally flowingin the anode circuit of valve 68 is abruptly reduced and then abruptlyincreased when the impulse of current through valve 33 ceases. Thedecrease oi current flo wing' in the anode circuit of valve 38 raisesthe potential of connection point 685 for the duration of the impulse ofvoltage on the grid, and a positive impulse of current is transmittedthrough condenser I2, rectifier 13, resistor 14, and conductor 15- tothe terminal 483 of condenser 48.

and H are charged so that the voltage of term-' Condensers l8 inal 48ais increased positively. This increase of positive voltage raises thegrid voltage of amplifier valve H, which results in increasing the anodecurrent of valve 31 flowing through resistor l8 and lowers the gridvoltage of valves 23, 24, and 25. This results in decreasing the voltageof generator 2i and decreasing the speed of correcting motor is whichincreases the speed of perforating rolls 23. The speed of the correctingmotor, and therefore the amount of the correction, is dependent on thepolarity and. magnitude of the charge on condensers l1 and t.

As long as the error in register exists, electrical impulses willcontinue to be applied simultaneously to the grids Na. and b of valve H,and positive current impulses will continue to be transmitted to thecondensers H and 48 in rapid succession, i. e., each time a registermark on the material passes beneath the scanning head. The magnitude ofthe impulse which results from the combined action of the two scanningheads and which is applied to charge the condensers ll and 48 varieswith the magnitude of the error in register. This is because the lengthof time the indication from the register marl; overlaps the indicationfrom the slit varies with the magnitude of the error in register. Thisis a very desirable feature because it contributes to the stability ofoperation of the correcting system. At any instant the correcting motorruns at a speed dependent upon the charge on the condensers. In otherwords, the correction which is being applied at any instant depends uponthe past history of the indications of error. Thus, instead of workingon an intermittent basis, the correcting equipment operates continuouslyas long as any error in register exists.

When impulses are transmitted to the condensers 41 and 48 as describedin the foregoing,

a high resultant voltage exists across the con densers for a short timeafter the charge is placed on the condensers. the relatively low timeconstant of condenser 48 and resistor 49;, the condenser 48 dischargesrapidly and its voltage decays rapidly. Consequently the resultantvoltage of both condensers decreases rapidly to a value near the valuewhich existed before the charge was put on but slightly increased withrespect thereto. The result of this action is that at first the speed ofthe correcting element 95;; of the differential is changed a relativelylarge amount for a relatively short time, and thereafter its speed isreturned to a However, because of I value which is only slightly changedfrom its original value and at which it continues to run until asubsequent correction becomes necessary. Thus, there will result theaction of a temporary speed increase or decrease for a positioncorrection combined with a follow up or velocity correction. This methodof correcting is very effective in preventing hunting.

A rectifier valve 11 has its anode connected to negative supplyconductor and its cathode connected to the anode of rectifier valve 13.

After an impulse has been transmitted through If the opposite error inregister should occur, 1. e., if the perforator rolls should become fastwith respect to material In, impulses from both scanning heads will besimultaneously received on the grids l5. and 45a of mixing valve 45.Each of these impulses will reduce the potential of connection point 45.below the potential of terminal 48 of condenser 48 and current will flowduring each impulse from terminal 48- through conductor 15, rectifiervalve '18 and condenser 18 and valve 45 to ground 58. In consequence,negative impulses are transmitted to condensers 41 and 48 to reduce thecharge thereon or possibly reverse the polarity, and ultimately toincrease the speed of correcting motor I! and correcting element 6adriven thereby. This will decrease the speed of the perforating rollsand bring them into correct register with the stamp face markings asexplained in the foregoing.

It should be noted that the cutting cylinders H are connected throughbevel gearing 19, shaft 80, and bevel gearing 8| to the perforatingcylinders and is therefore driven synchronously with the perforatingcylinders l3. Consequently, when the perforating cylinders are incorrect register with the stamp faces on the strip, the cuttingcylinders out of! sheets of stamps in correct register with the stampfaces. When the perforating cylinders are out of register, the cuttingcylinders are likewise out oi register and the operation of the registercontrol system to restore correct register oi the perforating cylindersresults in restoring correct register of the cutting cylinders.

Although in accordance with the provisions of the patent statutes, thisinvention is described as embodied in concrete form and the principle ofthe invention has been explained together with the best mode in which itis now contemplated applying that principle, it will be understood thatthe elements and connections shown are merely illustrative and that theinvention is not limited thereto since alterations and modificationswill readily suggest themselves to persons skilled in the art withoutdeparting from the true spirit of this invention or from the scope ofthe annexed claims.

What we claim as new and desire to secure by Letters Patent of theUnited States, is:

1. A control system for apparatus having an element operating on alength of moving material comprising light sensitive means actuated byindicia on said material for producing current variations, a secondlight sensitive means for producing current variations in accordancewith the position of said element, means responsive to the substantiallysimultaneous occurrence of said current variations for correcting therelative positions of said material and said element and electric valvemeans provided with a pair of control grids and requiring thesubstantially simultaneous variation of the voltages of both said gridsto change the conductivity of said valve means and electricalconnections from said first light sensitive means to one of said gridsand from said second light sensitive means to the other of said grids.

2. A register control system for apparatus having an element operatingon a length of moving material comprising an electric valve providedwith a pair of control members, said valve being normally non-conductingand being rendered conducting by the substantially simultaneousapplication or voltage to said control members, light sensitive meansresponsive to said material for applying voltage to one of said controlmembers, light sensitive means actuated by said element for applyingvoltage to the other of said members, and means controlled by saidelectric valve for correcting the relative positions of said materialandelement.

3. A register control system for apparatus having an element operatingon a length of moving material comprising in combination, an electricvalve having a pair of control members, said valve requiring thesubstantially simultaneous variation of the voltage of said controlmembers to eifect a change in its conductivity, light sensitive meansactuated by indicia orr said material for applying a voltage to one ofsaid control members, a second light sensitive means actuated by saidelement for applying a voltage to the other of said control members, thearrangement of said light sensitive means being such that both saidlight sensitive means apply vol tages to said control memberssubstantially simul taneously when an error exists in the relationshipof said element and material, and means responsiveto a change in theconductivity oi said valve for correcting said error.

4. A register control system for apparatus having an element operatingon a length of moving material comprising a pair of electric valves eachhaving a pair of control members, each of said valves requiring thesubstantially simultaneous application of voltage to both of its saidcontrol members to effect a change in its conductivity, light sensitivemeans responsive to indicia on said material for applying a voltage toone control member of each of said valves, light sensitive meansactuated by said element for applying voltage to the other controlmember of each of said valves, said light sensitive means being soarranged that when an error exists in the relative positions of saidelement and said length of material, voltage is applied simultaneouslyto both control members 01' one of said valves depending on thedirection of said error, and means responsive to a change in theconductivity oi said valves for correcting said error.

5. A register control system for apparatus having an element operatingon a length of moving material comprising an electric valve providedwith a control grid and a suppressor grid, said valve being normallynonconducting and being rendered conducting in response to thesubstantially simultaneous applicationof control voltages to both saidgrids, light sensitive means responsive to indicia on said material forapplying a control voltage to one of said grids, light sensitive meansactuated by said element for applying a control voltage to the other ofsaid grids, said light sensitive means being arranged so that saidcontrol voltages are supplied substantially simultaneously only when anerror exists in the relative position." or? said element and saidmaterial, and means responsive to the passage 01 current of said valvefor correcting said error.

6. A register control system for apparatus having an element operatingon a length of moving material comprising an electrical energy storagedevice, means responsive to relative positions of said material andelement for controlling the amount or electrical energy stored in saidenergy storage device, and means responsive to the amount of energystored in said energy storage device for correcting the relativepositions of said length of material and said element.

7. A register control system for apparatus having an element operatingon a length of moving material comprising a condenser, light sensitivemeans responsive to the relative positions of said element and materialfor controlling the charge on said condenser, and means responsive tothe charge on said condenser for correct i ing an error in the relativepositions 01' said element and said length of material.

8. A register control system for apparatus having an element operatingon a length 01 moving material comprising an electrical energy storagedevice, means responsive to the relative positions of said material andelement for causing an amount of electrical energy varying with themagnitude of the error in register between said material and element tobe stored in said energy storage device, and means responsive to theamount of energy stored in said energy storage device for correcting therelative positions of said length of material and said element.

9. A register control system for apparatus having an element operatingon a length of moving material comprising in combination, a condenser,light sensitive means responsive to the relative positions of saidelement and length of material for increasing the charge on saidcondenser in response to an error in one direction in the relativepositions of said element and length of material and for decreasing thecharge on said condenser in response to an error in the oppositedirection in the relative positions of said element and material, andmeans responsive to the charge on said condenser for correcting saiderrors.

10. A register control system for apparatus having an element operatingon a length 01' moving material comprising in combination, an energystorage device, light sensitive means responsive to the relativepositions of said material and said element for controlling the amountof energy stored in said energy storage device, means responsive to theamount of energy stored in said device for correcting an error in therelative positions of said element and material, said energy storagedevice comprising a portion having a relatively short time constant-anda second portion having a relatively long time constant thereby toprovide a relatively large temporary correction and a small relativelypermanent correction inthe relative positions 01' said element and saidmaterial.

11. A register control system for apparatus having an element operatingon a length 01' moving material comprising in combination, an electricvalve provided with a pair of control members, said valve requiring thesubstantially simultaneous application of control voltages to saidcontrol members to effect a change in the conductivity of said valve,light sensitive means responsive to indic'ia on said material forapplying control voltage to one oi said control members, meansresponsive to operation of said element for applying a control voltageto the other of said control members, said light sensitive means andelement responsive means being arranged so that said control voltagesare applied substantially simultaneously to said control members when anerror exists in the relative positions of said element and said materialthereby to effect a change in the current flowing through said valve,and means responsive to said change in current for correcting saiderror.

12. A register control system for apparatus having an element operatingon a length of moving material comprising a pair of electric valves eachprovided with a pair of control grids, each said valves requiring thesubstantially simultaneous application of control voltages to both saidgrids to effect a change in the conductivity of said valve, lightsensitive means responsive to indicia on said material for applyingcontrol voltages to one grid oi each of said valves, means responsive tosaid element for applying control voltages to the other grid of each ofsaid valves, said light sensitive means and said element responsivemeans being arranged so that when an error exists in the relativepositions of said material and said element said control voltages areapplied substantially simultaneously to the grids of one or the other ofsaid valves depending on the direction of said error thereby to effect achange in the current iiowing through said valve, and means responsiveto said change in current for correcting said error.

13. A register control system for apparatus having an element operatingon a length of moving material comprising in combination, an electricvalve having a pair of control grids, said valve requiring thesubstantially simultaneous application of control voltages to said gridsto effect a change in the conductivity of said valve, means responsiveto an error in the relative positions of said material and element iorapplying control voltages to both said grids simultaneously thereby toeffect a change in the current conducted by said valve, and meansresponsive to said change in current for varying the relative speeds ofsaid element and material to correct said error comprising a generatorhaving its voltages controlled in response to said change in current anda motor supplied from said generator.

14. A register control system for apparatus having an element operatingon a length of material comprising an electric valve having a pair ofcontrol grids, said valve requiring the substantially simultaneousapplication of control voltages to said grids to effect a change in theconductivity of said valve, means responsive to an error in the relativepositions of said element and material for applying substantiallysimultaneous control voltages to said grids to effect a change in thecurrent conducted by said valve, an electrical energy storage deviceconnected to said valve so that its stored energy is varied in responseto said current change, and means for correcting said error comprising agenerator connected to have its field excitation varied in accordancewith the amount of energy stored in said energy storage device and amotor supplied irom said generator.

15. A register control system for apparatus having an element operatingon a length material comprising in combination, means for feeding saidmaterial to said element, means for driving said element and feedingmeans at predetermined relative speeds, an electric valve having a pairof control grids, said valve requiring the substantially simultaneousapplication of control voltages to said grids to render said valveconducting, means responsive to an error in the relative positions ofsaid element and material for applying control voltages to said gridssub stantially simultaneously, a condenser connected to said valve sothat its charge is varied by the current conducted by said valve, andmeans responsive to the variation of the charge on said condenser forcorrecting said error at a rate dependent on said charge comprising agenerator having its excitation varied in accordance with said chargeand a motor supplied from said-generator and mechanically connected tosaid driv ing means.

16. A register control system for apparatus having an element operatingon a length of material comprising means ior driving said element andmaterial at predetermined relative speeds, an electric valve having apair of control grids, said valve requiring the substantiallysimultaneous application of control voltages to said grids to rendersaid valve conducting, means responsive to an error in the relativepositions of said element and material (or substantially simultaneouslyapplying control voltages to said grids to render said valve conducting,a pair of condensers connected in series relationship and connected tohave their charge varied in response to the current conducted by saidvalve, and means responsive to the variation in said charge for varyingthe relative speeds of said element and material to correct said errorat a rate dependent upon the magnitude of said charge, one of saidcondensers having a relatively large time constant and the other havinga relatively small time constant thereby to provide a temporaryrelatively large speed correction, and a small relatively permanentspeed correction.

17. A register control system ior apparatus having an element operatingon a length of material comprising means for driving said element andmaterial at predetermined relative speeds, a pair 01 electric valveseach having a pair of control grids, each of said valves being normallynonconducting and being rendered conducting by applying control voltagesto both its grids, light sensitive means responsive to an error in therelative positions of said element and material for substantiallysimultaneously applying control voltages to the grids of one of saidvalves to render one or the other 0! said valves conducting depending onthe direction of said error, a pair of condensers connected in seriesrelationship and connected with said valves so that the charge on saidcondensers is increased in response to the current conducted by one oisaid valves and decreased in response to the current conducted by theother of said valves, and means responsive to a variation in the chargeon said condensers for correcting said error, one of said condensershaving a relatively large time constant and the other having arelatively small time constant thereby to produce a relatively largetemporary correction and a small relatively permanent correction.

DAVID R. SHOULTS. WILLIAM D. 'COCKRELL.

